This application claims the priority of German Patent Application No. 197 43 959.4, filed Oct. 4, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a brake system for motor vehicles having a brake pedal, a brake booster, a master brake cylinder and an electronically regulatable brake unit, which is arranged between the master brake cylinder and the wheel brakes.
A brake system of this type is known, for example, by means of the DSC systems by BMW.RTM. which represent a further development of the ABS and ASC wheel slip control systems. A DSC system of this type by BMW.RTM. is described, for example, on Pages 134+ and Pages 208+ of Automobiltechnische Zeitschrift ATZ, 1997. Furthermore, a known brake system of this type is illustrated in FIG. 3.
A part of these known brake systems is a brake booster. When this brake booster fails, according to the acceptance requirements of type approvals (currently when a foot pressure of maximally 500 N is exceeded), a deceleration of 2.9 m/s.sup.2 must be achieved. This requirement can be met by the corresponding design of the hydraulic-mechanical transmission ratio (pedal ratio, master brake cylinder diameter). However, this design of the ratio conditions results in long pedal travels in the case of heavy vehicles and in an undesirably soft pedal sensation during normal braking. When the brake booster fails, the pedal forces required for a deceleration are so high that the driver may suspect a total failure of the brake system and may react inappropriately.
It is an object of the invention to improve upon the brake system of the above-mentioned type such that a satisfactory deceleration is achieved for the driver even when the brake booster has failed.
This object is achieved by a brake system for motor vehicles having a brake pedal, a brake booster, a master brake cylinder and an electronically regulatable brake unit arranged between the master brake cylinder and the wheel brakes. The brake pressure in the wheel brakes can be set independently of the preliminary pressure existing at the output of the master brake cylinder with an electronic control unit. In the event of a failure of the brake booster, by means of the electronic control unit and by means of the electronically regulatable brake unit, the brake pressure in the wheel brakes is generated such that a predetermined minimum desired deceleration value occurs as a function of a pedal operating value which is directly proportional to the pedal force applied by the driver by way of the brake pedal. Advantageous further developments of the invention are described herein.
Important aspects of the invention are the recognition of a failure of the brake booster and the generating of a brake pressure in the wheel brakes using the electronically regulatable brake unit such that, in the event of a defined pedal force applied by the driver by way of the brake pedal, a minimum deceleration occurs corresponding to a defined minimum desired deceleration course. This pedal force is determined directly (or from a pedal operating value which is directly proportional to the pedal force applied by the driver) by way of the brake pedal. Such a pedal operating value is preferably the preliminary pressure which exists at the output of the master brake cylinder and which normally is sensed anyhow with a pressure sensor in the hydraulic line by the electronic control unit assigned to the brake system. However, as an alternative, the direct measuring of the pedal force can also be done.
In the presence of defined pedal force or pedal operating value, the achieved actual deceleration is then determined in the electronic control unit and is compared with the desired deceleration corresponding to the predetermined minimum desired deceleration course. If the desired deceleration is larger than the actual deceleration, the electronically regulatable brake unit increases the brake pressure in the wheel brakes until the desired deceleration has been reached.
The predetermined minimum desired deceleration course is stored, for example, as a characteristic curve in the electronic control unit and is defined such that the acceptance conditions for a type approval are minimally met in the event of a failure of the brake booster.
The basic prerequisite for the brake system according to the invention is an electronically regulatable brake unit, such as the DSC hydraulic system, by which, independently of the pedal force applied by the driver by way of the brake pedal or independently of the preliminary pressure existing at the output of the master brake cylinder, the brake pressure can be adjusted in the wheel brakes.
The failure of the brake booster is preferably recognized in two different manners.
1. A failure of the brake booster exists, for example, when no vacuum exists in the brake booster. This is sensed by a differential pressure sensor on the brake booster which compares the pressure in the brake booster, for example, in its vacuum chamber, with the atmospheric pressure and emits a signal when there is no difference. When the differential pressure sensor, which may also be constructed only as a switch, indicates the presence of atmospheric pressure in the brake booster but simultaneously pressure is built up in the master brake cylinder by the operating of the brake pedal by the driver, the electronic control unit will conclude that there is a failure of the brake booster. This method recognizes the most frequently occurring cause of a failure due to a defective vacuum supply.
2. A failure of the brake booster will also exist when, although a vacuum is present in the brake booster, the intake of atmospheric air is prevented. In this case, a slightly lower pressure or a slightly higher vacuum may arise than in the vacuum chamber of the brake booster when the driver operates the brake pedal. At most, the same pressure or vacuum will arise in the working chamber as in the vacuum chamber. However, when the system is intact, the pressure in the working chamber will normally always be higher than in the vacuum chamber when the brake pedal is operated. The pressure difference between the pressure in the working chamber and the pressure in the vacuum chamber can be detected by a differential pressure sensor between the working chamber and the vacuum chamber.
When a failure of the brake booster is recognized, in addition to generating the brake pressure by the electronically regulatable brake unit, the driver can also be warned.
As a result of the brake system according to the invention, the driver can still brake by means of reasonable pedal forces in the event of a failure of the brake booster.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.